Hydrogen production is a multi-million dollar industry supplying high purity hydrogen for chemical producing industries, metals refining, petroleum refiners and other related industries. A typical commercial source for the production of hydrogen is the reforming of natural gas or other methane-rich hydrocarbon streams. The reforming is carried out by reacting the hydrocarbon with steam and/or with oxygen-containing gas (e.g. air or oxygen-enriched air), producing a hydrogen containing gas stream with accompanying amounts of oxides of carbon, water, residual methane and nitrogen. Unless it is desired to recover carbon monoxide, it is customarily converted to carbon dioxide by water gas shift (WGS) reaction to maximize the hydrogen content in the stream. Typically, this gas stream is then purified by adsorbing impurities using a regenerable solid adsorbent, usually regenerating the adsorbent by pressure swing adsorption (PSA) in a PSA unit. The PSA vessels generally contain a layer of activated carbon, for bulk CO2 removal, followed by molecular sieve for CO and N2 removal. A layer of activated alumina is sometimes used at the feed end of the bed for moisture removal. Other hydrogen-rich gas sources which can be upgraded by PSA technology to a high purity product include refinery off-gases with C1-C6 hydrocarbon contaminants and effluent streams from partial oxidation units.
Precursors for hydrogen other than natural gas can be used for example coal, petroleum coke, biomass and other cheap precursors. The production of hydrogen from coal or petroleum coke typically would involve gasification or partial oxidation of the solid material. This gasification step combines coal, oxygen and steam at high temperature and pressure to produce a synthesis gas. The resultant synthesis gas can be treated by the water gas shift reaction (CO+H2O═CO2+H2) to supplement hydrogen production.
The synthesis gas derived from gasification processes using coke, petroleum coke or biomass is inherently different from the synthesis gas produced by steam reforming of hydrocarbons like natural gas. In the case of steam reforming of natural gas, the resultant synthesis gas is very clean and contains a few impurities in the hydrogen-rich feed stream to the PSA. In the case of gasification-derived synthesis gas, the gas contains numerous impurities including sulfur species (H2S, COS, mercaptans), metals (Hg), various chlorides, carbonyls (Ni and Fe carbonyl), arsenic, heavy hydrocarbons, ammonia, HCN, olefins, diolefins, acetylenics and aromatics. The presence of these species in the synthesis gas presents a problem for the PSA system. Some of these species like carbonyls, heavy hydrocarbons and aromatics will be very strongly adsorbing and hence difficult to desorb. If these strongly adsorbing components do not desorb during the regeneration step, the capacity of the PSA decreases and the hydrogen production rate of the PSA decreases. Other species, e.g. the sulfur species, can react with the adsorbent surface resulting in sulfur plugging of the adsorbent (and consequent loss in adsorption capacity). This is especially a problem with H2S since the concentration of this impurity in gasification-derived synthesis gas can be up to 5 vol %.
Other important aspects that need to be considered with non-natural gas derived synthesis gas are as follows. Small, unreacted amounts of oxygen may be present in the synthesis gas stream. The presence of oxygen in the synthesis gas stream greatly enhances the amount of sulfur deposition on adsorbents via the reaction H2S+½O2=S+H2O. Also, nitrogen in significant concentrations can be present in the synthesis if air is used for the oxidation process. To produce high purity hydrogen, this nitrogen must be removed by the PSA. For this nitrogen removal step, a zeolite adsorbent is required. Since H2S is very strongly adsorbed on zeolites, care must be taken in the PSA design to ensure H2S does not reach the N2-removing zeolite layer. Finally, CO removal from the synthesis gas will be required. CO removal will require a zeolite adsorbent which as in the case of N2 removal must avoid contact with H2S.
Gasification systems have also recently been considered for clean power production with reduced CO2 emissions. The solid carbonaceous fuel is gasified to synthesis gas, shifted in a sour WGS reactor, cooled, and separated into a CO2/H2S containing stream and a decarbonized H2 product stream. The latter is combusted with air or oxygen-enriched air in a gas turbine to produce power with essentially N2 and H2O in the vent stack. In this case, high purity H2 (99.9+%) is not necessary. Generally the process goal is to remove 50-90% of the carbon species in the syngas feed—some of the carbon species can be tolerated in the product H2 gas. High recovery of H2 is critical for successful implementation of this approach since it impacts the solid fuel feed rate to the gasifier and hence the size of all of the equipment from gasifier to H2 PSA. Integration of the separation process with the rest of the power generation process is also vital.
In most gasification to hydrogen schemes an acid gas removal system is employed prior to the PSA. This acid gas removal step (typically done by absorption with amines, cold methanol, glymes etc.) keeps H2S away from the PSA ensuring a robust system. If the gasification-derived synthesis gas could be fed directly into a PSA, then the cost and expense of the acid gas removal system could be avoided.
Typical H2 PSA systems used for upgrading synthesis gas derived from natural gas will quickly lose performance over time (lower H2 production rate, lower H2 recovery) if used in the same way to purify gasification derived synthesis gas owing to the different impurities present.
Thus, it would be desirable to provide a robust adsorption system which can tolerate all the impurities present in gasification-derived synthesis gas.
Previous proposals for dealing with the production and purification of H2 by pressure swing adsorption (PSA) from gas streams that contain significant amounts (1 vol % and higher) of sulfur containing species like H2S primarily fall into two categories: 1) art which shows that H2S and acid gases should be removed prior to the PSA and 2) art which suggests that H2S can be put directly into a PSA system, although generally without addressing specific issues relating to the stability or longevity of the process.
U.S. Pat. No. 4,553,981 teaches a process to produce high purity H2 (99.9+%) from gas streams obtained by reforming of hydrocarbons, partial oxidation of hydrocarbons and coal gasification. The system consists of a synthesis generator (e.g. a gasifier), a water gas shift reactor (to convert CO and H2O to CO2 and H2), a liquid scrubber and a PSA system. The liquid scrubber is used to remove acid gases (like CO2 and H2S) from the feed stream prior to the PSA. Other references that suggest H2S removal should be accomplished prior to introduction into the PSA include U.S. Pat. No. 5,536,300; GB 2,237,814 and WO 2006/066892.
U.S. Pat. No. 4,696,680 teaches putting an H2S containing feed directly into a PSA bed. It is said that H2S can be selectively and reversibly removed from coal-derived synthesis gas using either activated carbon and/or zeolite adsorbents.
US 2002/0010093 appreciates the fact that reaction of activated carbon with H2S can occur in H2 PSA processes. To obviate this, the activated carbon is acid washed prior to use. The acid washing step removes inorganic impurities which may help catalyze the formation of elemental S in the adsorbent pores. This reference also teaches a layered bed approach to H2 production from an H2S containing stream which consists of a first layer of alumina or silica gel, a second layer of acid washed carbon and a final layer of zeolite.
U.S. Pat. No. 5,203,888 teaches a pressure swing adsorption process for the production of hydrogen where H2S could be present in the feed gas and that suitable adsorbents include molecular sieves, carbons, clays, silica gels, activated alumina and the like. U.S. Pat. No. 6,210,466 similarly teaches that H2S can be put directly into a PSA to produce purified methane.
EP 486174 teaches a process for producing hydrogen via partial oxidation of various hydrocarbon feedstocks (e.g. refinery off-gas). The synthesis gas produced by this process could contain high levels of H2S (up to 4 vol %). The synthesis gas produced is passed directly into a PSA for H2 purification. There is no reference to the preferred PSA cycle or adsorbents required.
US 2005/0139069 teaches a process for the purification of a hydrogen stream that contains H2S. The adsorbent materials cited for the application include carbon, zeolite, alumina and silica gel. The PSA is coupled with an integrated compressor for recycle of purge or residual gas to the hydrodesulfurization process.
U.S. Pat. No. 4,696,680 states as adsorbents activated carbons, zeolites, or combinations thereof. Izumi et al (Fundamentals of Adsorption; Proc. IVth Int. Conf. on Fundamentals of Adsorption, Kyoto, May 17-22, 1992) concludes that the best H2S adsorbent is silicalite or alumina.
U.S. Pat. No. 7,306,651 states that the H2PSA beds should consist of at least two adsorbents chosen from activated carbons, silica gels, aluminas or molecular sieves, preferably with ‘a protective layer composed of alumina and/or silica gel at the feed end of the bed.
U.S. Pat. No. 5,797,979 teaches the separation of H2S from gas streams using ion exchange resins. Useful materials are macroreticular anion exchange resins containing a basic anion for which the conjugate acid has a pKa value ranging from 3 to 14. Specific examples are the fluoride or acetate form of Amberlyst A26 resin. The resin contains a quaternary ammonium moiety and either fluoride or acetate counterions. A cited H2S capacity for the fluoride containing resin was 1.0 mmol/g at 25 C and 0.05 atm H2S. Adsorption occurs via a chemical reaction between H2S and the basic anion as described in Sep. Sci. Tech, 38, 3385-3407 (2003). Regeneration of the H2S free adsorbent was accomplished by heating to 50 C while purging with inert gas, humidified inert gas, or dynamic vacuum.
A PSA or other swing adsorption purification of hydrogen would normally be operated using hydrogen as a purge and repressurisation gas. The beneficial use of nitrogen purge or repressurization has been proposed. U.S. Pat. No. 4,333,744 describes a ‘two-feed PSA process’ in which a portion of the PSA feed gas is first sent to a CO2 separation unit and the CO2-lean product gas is processed in the PSA followed by the remaining PSA feed gas. N2 can be used as a purge gas or a repressurization gas to form an ammonia synthesis gas.
U.S. Pat. No. 4,375,363 described the use of nitrogen purge and repressurization in a typical PSA cycle to produce a nitrogen/hydrogen product used for ammonia synthesis. High pressure nitrogen is used to help displace hydrogen from the bed after the feed step, again for the production of ammonia synthesis gas. U.S. Pat. No. 4,414,191 extends this approach by utilizing a nitrogen purge step at elevated pressure, to incorporate more of the nitrogen in the H2 product.
U.S. Pat. No. 4,578,214 utilized a nitrogen purged PSA unit integrated with a fuel cell system to produce ammonia synthesis gas. The fuel cell provides electrical power and supplies the source for the N2 stream (O2-depleted air).
U.S. Pat. No. 4,813,980 describes production of ammonia syngas via a PSA process utilizing two sets of adsorber beds, one to remove CO2 and the second to remove other impurities, from a feedstock consisting of bulk H2, CO2, and N2 and <10% other impurities. The beds of the second set are purged and repressurized with a nitrogen-containing gas. This gas could be a portion of the N2/H2 product, a recycle stream from the ammonia process, or N2 obtained from other sources.
U.S. Pat. No. 4,696,680 describes the use of a guard bed for hydrogen sulfide removal upstream of a separate vessel for PSA purification of hydrogen by the removal of other impurities. The guard bed contains activated carbon, zeolites, or combinations thereof for removing both hydrogen sulfide and carbon dioxide.
WO2005/118126 teaches a bed of chemisorbent (e.g. ZnO) as a guard bed to remove H2S prior to a H2PSA. The ZnO bed works by reaction of with H2S and is not a regenerable bed. Also, the H2S concentration in the feed gas in '126 is only in the ppm range because the source of H2 is natural gas.